1. Make sure you're using the correct formula for the dimensions given. The usual formula gives volume in terms of radius and height. If you are given the diameter, the formula will be different, or you need to compute the radius before you use the formula.
2. Make sure you're using the appropriate value for π. Many calculators have the value built-in. Many problems posted on require the use of 3.14, which will give different answers. (One recent problem required the use of 3.) If your calculator doesn't have π built in, a reasonable value is 355/113, which is good to 7 significant figures.
3. Make sure the units you are using are compatible (generally, all the same). If your height is in one unit (say inches) and your diameter is in another unit (say centimeters), you need to do units conversion before you put the numbers in the formula. The result of putting your units in the formula with your numbers should be that you end up with units cubed. For example, for a radius of 2 cm and a height of 3 cm, the volume will be
.. V = π(2cm)^2*(3 cm) = 12π cm^3.
4. Compare the dimensions and the volume to things you know. You know the approximate size of a gallon jug, a 2 liter pop bottle, a 5-gallon bucket. Check your answer for reasonableness.
5. Make an estimate based on the dimensions. Round to 1 or 2 significant figures and make a guess as to the approximate result you should get. For this, you can use 3 for π, as you just want to be "somewhere in the ballpark" as opposed to being off by a factor of 10 or more. This requires a certain amount of number sense and knowledge of multiplication tables.
6. Make certain your calculator is being used correctly. If parentheses are involved, make sure you enter the closing parentheses--as opposed to letting the calculator put them in according to its own rules. If division or fractions are involved, make sure you have parentheses around the denominator in every case. 1/2*3 ≠ 1/(2*3) It can be helpful to use a calculator that shows you what it did. (The Google calculator does that, for example.)
7. Sometimes, it helps just to do the calculation twice (possibly in a different order). Inadvertent error can creep in even when you think you're paying attention.
8. If you're doing the math by hand, make use of all available techniques for checking your arithmetic.
